1. Field of the Invention
The present invention relates to a knock control apparatus for an internal combustion engine which detects the presence or absence of the occurrence of knocking (hereinafter abbreviated as a “knock”) based on the waveform of an ionic current generated upon combustion of the internal combustion engine.
2. Description of the Related Art
In general, ions are generated when fuel is burned or combusted in cylinders of an internal combustion engine, so an ionic current can be observed by a probe installed in each cylinder with a high voltage impressed thereto. Also, it is known that noise of a vibration component of the same vibration frequency as that of a knock is superposed on such an ionic current. Accordingly, in the past, there has been proposed an apparatus that detects the occurrence of a knock by extracting a vibration component superposed on an ionic current and performs knock control (retard correction of ignition timing) (see, for instance, a patent document: Japanese patent application laid-open No. H10-9108).
In the known knock detection apparatus for an internal combustion engine disclosed in the above-mentioned first patent document, a high voltage for observation of an ionic current is produced by using a secondary voltage generated in an ignition coil at the time of ignition thereof, and an ionic current is measured by impressing the high voltage thus produced to an end of a spark plug immediately after the termination of a discharge thereof. A knock signal in the form of a vibration component of a knock frequency band (knock signal) is extracted from the ionic current through a band-pass filter, and the knock signal thus extracted is amplified and waveform shaped by comparison with a detection threshold to produce a knock pulse.
Hereinafter, the knock pulse is sent to an engine control unit (ECU) where it is used for a determination as to whether a knock occurs, and the ECU performs ignition timing retard control upon the occurrence of a knock based on this determination result.
However, noise of the same vibration component as a knock frequency might sometimes be superposed on an ionic current, depending upon the operating state of the internal combustion engine, in spite of the non-occurrence of a knock.
In addition, it is known that some engines generate pressure pulsation in cylinders without regard to the occurrence of a knock, and a vibration component might superpose on an ionic current waveform upon generation of such pressure pulsation.
In the known knocking detection apparatuses for an internal combustion engine, there is the following problem. That is, a knock vibration component is extracted by using a band-pass filter, etc., so a noise component having the same frequency component as a knock frequency can not be distinguished from a knock.
In particular, in a knock detection apparatus based on an ionic current signal (detection signal for an ionic current), a noise component having a vibration amplitude intensity and a vibration duration comparable to a vibration signal generated upon the occurrence of a large knock is superposed on the ionic current signal, so there is a problem that it becomes very difficult to extract only a knock signal from the ionic current signal.